1. Field of the Invention
This invention relates to a semiconductor laser chip. The present invention also relates to a laser pickup device which incorporates a semiconductor laser chip. The invention further relates to a method of making a semiconductor laser chip.
2. Description of the Prior Art
As is well known, a laser pickup device is used for optically reading information from an optical data carrier such optical audio disk or optical video disk. Compared to conventional analog pickup devices wherein the pickup comes into direct mechanical contact with a data carrier, the laser pickup device provides a longer life and prevents damage of the data carrier disk thanks to the absence of direct mechanical contact.
FIG. 14 of the accompanying drawings shows a typical arrangement of a prior art laser pickup device. As shown, the pickup device comprises a laser device LD and an optical system 11a. The laser device LD includes a semiconductor laser chip SLC1 for laser beam emission.
As shown in FIG. 15, the laser chip SLC1 has a stripe 10a (gain region) which extends perpendicularly to an output cleavage face. Thus, the laser chip emits an output laser beam 12a which is perpendicular to the output cleavage face.
The optical system 11a includes a diffraction grating 15, a beam splitter 16, an object lens 17, a concave lens 18, a cylindrical lens 19 and a photosensor 20. The diffraction grating 15 diffracts the output laser beam from the laser chip SLC1 into a main beam P.sub.0 and two auxiliary beams P.sub.1, P.sub.2. The diffracted beams pass through the beam splitter 16 and the object lens 17 for reflection on an optical data carrier disk D. The reflected beams P.sub.0 ', P.sub.1 ', P.sub.2 ' return along the reverse path through the object lens 17. A major portion of the reflected beams P.sub.0 ', P.sub.1 ', P.sub.2 ' is deflected by the beam splitter 16 and passes through the lenses 18, 19 for optical reading by the photosensor 20.
On the other hand, a minor portion of the reflected beams P.sub.0 ', P.sub.1 ', P.sub.2 ' passes through the beam splitter 16 for returning to the laser chip SLC1. The returning beam portion 13a (FIG. 15) is reflected by the output cleavage face of the laser chip SLC1 for re-entry into the optical system 11a. Such re-entry of the returning beam portion is known to cause noises (called "optical feedback induced noise"), consequently deteriorating the performance of the laser pickup device.
Thus, it has been conventionally proposed to provide a low reflective coating on the output cleavage face of the laser chip or otherwise treat the output cleavage face for reducing the optical feedback induced noise. However, such a solution requires an additional process step, thus resulting in a cost increase. Further, the conventional solution does not necessarily result in complete elimination of the return beam noises.